Accurate and rapid collection and distribution of geophysical property data is a key to successful exploration and production of petroleum resources. Based on data such as electrical and nuclear properties collected in a well-bore, as well as the propagation of sound through a formation, geophysicists make an analysis useful in making many important operational decisions. The analysis includes determination of whether a well is likely to produce hydrocarbons, whether to drill additional wells in the vicinity of an existing well, and whether to abandon a well as being unproductive. Geophysicists may also use well-bore data to select where to set casing in a well and to decide on how to perforate a well to stimulate hydrocarbon flow. One method of collecting well-bore geophysical properties is by way of wireline well-logging. In wireline well-logging, a well-logging tool (also often referred to as a sonde) is lowered into a well-bore on an electrical cable, the wireline. The well-logging tool is an electrically powered measurement device that may, for example, collect electrical data, sonic waveforms that are propagated through the surrounding formation, or radioactivity counts. These measurements are usually converted to a digital form and transmitted on the wireline. Systems for transmitting data from the well-logging tool to a surface data acquisition system over a wireline cable are known as wireline telemetry systems.
One prior art wireline telemetry system is the Digital Telemetry System (DTS) of Schlumberger Technology Corporation. U.S. Pat. No. 5,838,727 (hereinafter, 727; incorporated herein by reference) describes DTS. Another wireline telemetry system is described in co-pending U.S. patent application Ser. No. 09/471,659 which is incorporated herein by reference.
Wireline cables are primarily designed for mechanical strength and power delivery. A modern oil well may be drilled to a depth of in excess of 30,000 feet. The cable must be able to sustain the tension generated from the weight of the logging tools and the weight of the lengthy cable itself. The cable must also deliver relatively large quantities of power by alternating current or direct current to the toolstring. High frequency signal transmission properties, on the other hand, are given a lower priority. Therefore, wireline cables are not ideal conveyors of the information that is transmitted from the well-logging tools. It is desirable to provide wireline telemetry systems that can be tailored for specific or individual cables and conditions to maximally use the data delivery capabilities of a specific wireline cable.
Using a formula, known as Shannon's capacity formula, it is possible to determine a theoretical maximum channel capacity of a communication channel given a certain level of noise. Prior art well-logging telemetry systems achieve data rates that are considerably lower than the theoretical capacity. While it may not be practical (or even possible) to build a system that does achieve the Shannon capacity, it is nevertheless desirable to provide a system that achieves a data rate that comes as close as possible to the Shannon capacity for a given wireline cable.
Because of the electrical limitations on a wireline cable, the signal-to-noise ratio can be unacceptably high and significantly impact the data rate. It would be desirable to provide a system and method which overcomes the signal-to-noise ratio problems associated with wireline telemetry systems.
Modern wireline cables contain several electrical conductors, for example, 7 wires and the outer armor. Data can be simultaneously transmitted on these several conductors. The distinct combinations of conductors used are referred to herein as “propagation modes”. Far-end cross-talk between the several propagation modes used simultaneously is a significant source of noise in data transmission. Far-end cross-talk is the interference between data transmitted in one propagation mode and the data transmitted in another propagation mode. Far-end cross-talk is caused by imperfections in the symmetry or insulation of the wireline cable, as well as circuitry that is used for interfacing to the cable downhole and at the surface. Far-end cross-talk impacts both data rate and robustness of the data transmission. Cross-talk limits the available data rate and reliability. For example, cross-talk can lead to transmission failures during the progress of a logging job.
Hitherto the impact of far-end cross-talk has been avoided by precise cable design or by decreasing data rate. For example, cross-talk may be avoided by requiring near perfect electrical insulation, perfect geometry and near perfect conduction properties. Naturally, these requirements increase the cable cost and also causes the need to decommission cables relatively early due to wear. Furthermore, cross-talk may occur at the cable heads. Therefore, there is also a requirement to maintain very high insulation standards at the cable heads. Doing so can be very difficult in the harsh conditions encountered in logging jobs, e.g., high temperature and pressure.
An alternative approach to reduce the impact of far-end cross-talk is to reduce the data rate. At lower data rates the data transmission is more resilient to noise, including the noise produced by cross-talk. However, having lower data rates increases the time required for logging a well and therefore the costs associated with the logging operation and the costs due to putting other operations on hold while the well is being logged.
From the foregoing it will be apparent that there is still a need for a way to minimize the impact that far-end cross-talk has on throughput and reliability in a wireline telemetry system.